In automobile engines, a cooling system of a water cooling type employing a radiator is typically used to cool the engine. In the cooling systems of this type, a thermostat using a thermally expandable body for adjusting the amount of cooling water circulating on the radiator side has been conventionally used, so that the temperature of the cooling water introduced into the engine could be controlled.
Thus, a thermostat using the aforesaid thermally expandable body is installed in part of a cooling water passage, for example, on the inlet side or outlet side of the engine, and the temperature of cooling water can be controlled to the prescribed level by closing the control valve and causing the cooling water to circulate via a bypass passage, so that it does not pass through the radiator, when the temperature of cooling water is low, and opening the control valve and circulating the cooling water via the radiator when the temperature of the cooling water becomes high.
Incidentally, it is well known that in automobile engines, a fuel consumption ratio is increased if mechanical friction of the engine is reduced. Various research and developments have been conducted to improve fuel consumption based on this well-known technology.
The increase in the wall temperature of a cylinder block is most closely related to friction reduction allowing fuel consumption to be improved, and employing independent cooling water circulation paths for a cylinder head and a cylinder block and increasing the cylinder block wall temperature, without circulating the cooling water in the cylinder block during warm-up, was confirmed to be important for accelerating the increase in the cylinder block wall temperature.
A dual-circuit cooling system in which cooling water control of the engine is conducted in the above-described two circuits makes it possible to meet such a requirement. Thus, the dual-circuit cooling system comprises independent cooling water circulation passage of the cylinder head of the engine and a cooling water circulation passage of the cylinder block, and when the temperature of cooling water is low, the circulation of cooling water is carried out only in the passage on the cylinder head side, whereas when the temperature of cooling water becomes high, the cooling water is circulated in the passages of both the cylinder head and the cylinder block.
With such a dual-circuit cooling system, the cylinder block wall temperature can be raised by reducing the flow rate of cooling water on the engine block side. Accordingly, the aforementioned effect has been conventionally obtained by using two thermostats with different temperature characteristics and conducting temperature control of cooling water passages in the two circuits.
For example, Japanese Patent Application Laid-open No. H5-215008 disclosed a cooling system for an engine, which employs the above-described thermostat for a dual-circuit cooling system, wherein two thermostats are used and the cooling water in the cooling water passages for cooling the cylinder block and the cylinder head can be controlled independently.
Japanese Patent Application Laid-open No. S57-97014 discloses a configuration in which one thermostat is used and cooling water in the passages of two circuits can be controlled.
However, in the cooling system described in Japanese Patent Application Laid-open No. H5-215008 two cooling systems for the cylinder head and the cylinder block were provided independently, and those passage systems were controlled independently. Therefore, two or more thermostats were needed or control valves had to be provided in separate locations. As a result the number of structural components was increased, the structure became complex, the cost was increased and the entire apparatus increased in size.
In the cooling system described in Japanese Patent Application Laid-open No. S57-97014, control of cooling water in the above-described two circuits was carried out by using one thermostat. Therefore, the thermostat structure was complex and the entire apparatus increased in size. In particular, in the above-described dual-circuit cooling system, cooling water systems of the two circuits had to be controlled independently and the control had to be conducted so that the temperature of the cooling water on the cylinder block side became higher than that of the cylinder head side. As a result, the structure was complex.
In particular, in the thermostat used in the above-described dual-circuit cooling system, the switching timing for connecting the passage on the above-mentioned cylinder block side is preferably operated with a shift with respect to the switching timing of the main valve in order to increase the temperature of cooling water on the cylinder block side over the temperature of cooling water on the cylinder head side. In other words, the structure is desired in which when the main valve controlling the opening and closing of the passage on the cylinder head side is closed, the subsidiary valve controlling the opening and closing of the passage on the cylinder block side is closed, and when the main valve is open, the subsidiary valve is open to match the prescribed lift of a piston rod.
Further, in a thermostat of a bottom bypass type that has been known conventionally, the first and second valves are linked so that only a transition from the open state to the close state is possible and cannot be directly used. Accordingly, taking the aforementioned issues into consideration, there is a need for providing a thermostat for a dual-circuit cooling system, this thermostat having a simple structure and allowing the desired operation state to be obtained.
With the foregoing in view, it is an object of the present invention to obtain a thermostat for a dual-circuit cooling system which allows the cooling water passages of the dual circuit to be controlled independently, makes it possible to conduct the control of cooling water in each cooling water passage at respective required timing, has a simple structure, allows the entire apparatus to be reduced in size, and is cost effective.